The present invention is directed to a directional coupler for coupling a light signal traveling in a light conducting core of one waveguide to the light conducting core of a second waveguide with an adjustable ratio of the amount of light coupled between the cores. The adjustable ratio in the amount of light, which is being coupled, is a division ratio of the coupler and is understood to be the ratio of the intensity of the light being coupled to the second waveguide relative to the intensity or amount of light remaining in the first waveguide.
In the employment of light waveguides for example in optical communication technology, devices are required in order to couple light into one optical waveguide partially or entirely from another optical waveguide such as between first and second optical fibers. This can occur for example if the optical fibers are laid next to one another and in contact along a certain length or coupling segment. In an article, B. S. Kawasaki and K. O. Hill, "Low-Loss Access Coupler for Multimode Optical Fiber Distribution Networks", Applied Optics, Vol. 16, No. 7, July 1977, pp. 1794-5, a coupler was described in which two fibers were twisted with respect to one another and are heated locally so that the fibers in the twisted zone are drawn up to a biconical form and are fused together. In this coupler 3.82% through 24% of the light traveling in one fiber is coupled over into the other fiber. However, it is difficult to reproducibly manufacture the coupler with a selected one of these division ratio and the ratio cannot be adjusted.
In an article by Alan L. Jones, "Coupling of Optical Fibers and Scattering in Fibers", Journal of the Optical Society of America, Vol. 55, No. 3, March 1965, pp. 261-71, the radiation of a light from one optical fiber into a second optical fiber laid parallel thereto is theoretically investigated. In the article it is shown that a complete coupling over is possible between the two identical waveguides, which have the same propogation constant, when the coupling segment corresponds to an uneven multiple of a coupling length L.sub.0. This coupling length L.sub.0 is all the greater when the distance between the cores of the light waveguides becomes greater. If a coupling segment L.ltoreq.L.sub.0, the coupling decreases exponentially when the relationship of the distance of the waveguides to the penetration depth of the electrical field increases in the medium connecting the waveguides. The penetration depth in turn depends on the difference between the index of refraction of the waveguide and the index of refraction of the surrounding medium and decreases as this difference increases. When observing a specific coupling constant and due to this property, a multimode of magnitudes must be held within narrow tolerances which raises technological problems in the manufacture of such couplers and these problems have not been solved up to now. At present, a subsequent adjustment of the directional coupler to a desired division ratio, for example and adjusted to a specific wave length or under certain conditions, a modulation of the division ratio, has been achieved up to now by changing the index of refractions.